Cloxure

A Clojure implementation of the Lox programming language from the amazing book Crafting Interpreters by Bob Nystrom.

This is a tree-walking implementation based on the reference jlox implementation written in Java.

I have tried to keep the code semantically as close to jlox as possible, but adapted to Clojure's functional style. This means that a lot of components from the interpreter don't use mutable state, but instead use a state passing pattern using immutable data structures. The exception to this is the code for storing scoped variables and object properties, which uses Clojure atoms and hence are mutable references. Implementing an interpreter for an imperative language such as Lox using purely functional code is possible, but it requires a lot of extra work, specially around handling lexical closures. So I decided to be pragmatic as Clojure itself and just use atoms for those parts.

Another difference from the original implementation, is that instead of using the visitor pattern for tree walking, I use the more natural multimethods. In other cases where polymorphism is needed, such as in LoxCallable, I have used a protocol instead.

The code passes the entire test suite form Lox. The test files have been copy pasted form the original Crafting Interpreters repository.

The benchmarks have also been copied. As expected, the performance of Cloxure is even worse than jlox. This is mostly due to the fact that on every evaluation there is a new copy of the state being created. Even with Clojure's persistent data structures, this is considerably slower than just using mutable state. Performance is not the goal of this implementation, but rather learn more about Clojure, interpreters and functional programming.

How to run

To compile and run this project you need Leiningen.

To run the REPL, just go for:

lein run

To run a specific file:

lein run program.lox

Running the tests

To run the tests just run:

lein test

Byte-code VM implementation

I also wrote another implementation of Lox using a byte-code VM Rust called Loxido.

Copyright note

Most of the test and benchmark files are copied from Crafting Interpreters repository. The copyright for these belongs to Bob Nystrom and are copied here because their license allows it (MIT).

The source code for this implementation is copyright of Manuel Ceron. And it's also licensed as MIT (see LICENCE for details.)